Servo stabilized meter apparatus

ABSTRACT

Disclosed is a closed loop system for providing precise, stabilized displacement of an indicator in accordance with command signal. A D&#39;&#39;Arsonval type meter structure has first and second stationary electrically conductive plates mounted on and insulated from the magnetic core structure. Each plate has a portion thereof disposed between the conductive bobbin on which the armature coil is wound and the magnetic core structure so that when the bobbin moves in either direction the capacitive coupling between the bobbin and one of the plates increases as the capacitive coupling between the bobbin and the other plate decreases. Means are provided for differentially sensing the capacitance between the bobbin and each of the two plates to develop an error signal proportional to armature displacement and for summing the error signal with the input or command signal to the armature coil to drive the indicator to the correct position.

nited States Patent [191 Runyan et al.

[ 51 Mar. 27, 1973 SERVO STABILIZED METER APPARATUS [75] Inventors:Wesley G. Runyon, Cedar Rapids; Lewis E. Staley, Marion, both of Iowa[73] Assignee: Collins Radio Company, Dallas,

Tex.

[22] Filed: Jan. 10, 1972 [21] Appl. No.1 216,515 1 [52] US. Cl...324/99, R, 317/246, 324/157 [51] Int. Cl. ..G0lr;17/06, H01g 5/10 [58]Field of Search.....324/99 R, 100, 157; 340/200;

[56] V 1 References Cited UNITED STATES PATENTS 1,970,442 8/l934Wittkuhns ct al. ..3l7/246 X 2,011,315 8/1935 Gilbert ....324/99 R2,117,894 5/1938 Lenehan.... .'.340/200 INPUT SIGNAL PrimaryExaminer-Rudolph V. Rolinec Assistant Examiner -Ernest F. KarlsenAttorney-Robert .1. Crawford et al.

57 ABSTRACT Disclosed is a closed loop system for providing precise,stabilized displacement of an indicator in accordance with commandsignal. A DArsonval type meter structure has first and second stationaryelectrically conductive plates mounted on and insulated from themagnetic core structure. Each plate has a portion thereof disposedbetween the conductive bobbin on which the armature coil is wound andthe magnetic core structure so that when the bobbin moves in .eitherdirection the capacitive coupling between the bobbin and one of theplates increases as the capacitive coupling between the bobbin and theother plate decreases. Means are provided for differentially sensing thecapacitance between the bobbin and each of the two plates to develop anerror signal proportional to armature displacement and for summing theerror signal with the input or command signal to the armature coil todrive the indicator to the correct position.

7 Claims, 4 Drawing Figures SERVO'STABIIJIZED METER APPARATUS Thisinvention relates to electrically actuated meters and moreparticularlyrelates' to a unique closed loop system for providingprecise, stabilized displacement of an indicator in accordance with acontrol signal.

Presently, in a variety of applications, metering and indicatingrequirements are' satisfied by variations of the basic .DArsonval metermovement. With this type of movement, signal current and a magneticfield produce a signal torque which causes a pointer to deflect andaspring to flex. A counter torque is developed by the meter spring as itflexes, and the pointer comes to rest when this counter torque balancesthe signal torque. The signal is then measured by the pointer's positionon the calibrated dial. Thus the correspondence between signalcurrentand dial indication (the measurement) is somewhat indirect, depending asit does upon magnetic field strength, a spring constant, and the dialcalibration.

In addition, the measurement may be affected by various erratic torquescaused by bearing friction and pointer adhesion. This is particularlytrue when signal current is kept low in order to minimize loading on thesignal source. Then the signal and spring torques are correspondinglylow, thusaccentuating the effect of er ratic torques. Considerable care,such as insuring cleanliness, is required to minimize erratic torques insensitive meters where pointer sticking must be avoided.

Most of the disadvantages of the DArsonval meter movement .can beovercome by the use of servo techniques, where the meter movement issensed and a signal proportional to the movement is fed back to themeter drive coil to drive the meter to the correct position. One such isdisclosed in U. S. Pat. No. 3,577,195 based on an induced magnetic fieldprinciple. Such servo meter systems, however, are generally complex inconstruction and expensive in implementation.

It is therefore an object of this invent-ion to provide a new meterapparatus, and a further object to provide a new servo stabilized metersystem which takes advantage 'of the novel aspects of the new meter.

In a preferred embodiment of the invention, a DArsonval type metermovement has a permanent magnet positioned relative to a soft ironmagnetic yoke ina manner to establish a DC magnetic field across an airgap between the yoke and the magnet in known fashion. An armature coilis wound on an electrically conductive bobbin and mounted within the airgap structure created between the permanent magnet and the soft ironyoke. Two appropriately-shaped electrically-conductive plates aremounted on and electrically insulated from the permanent magnet and eachother such that a varying portion of each plate lies underthe conductivebobbin when the drive' winding and the associated meter pointer arewithin the limits of the range of travel of the bobbin. Suitableelectrical connections are made to the bobbin and the plates to form twoprincipal capacitors. When a command signal is applied to the drive coilto cause movement of the coil and its associated meter pointer to thecommanded position, the shape of the plates and the bobbin movementcombine to cause the capacitance between the bobbin and one Positionsensing means are provided to differentially sense the change ofcapacitance of the two capacitors and therefore provide a displacementfeedback signal which is degeneratively combined with a command signalto drive the meter to the correct position.

The attributes of the invention will be best understood from aconsideration of the following specification with reference to thedrawings in which:

FIG. 1 illustrates in a simplified pictorial form a DArsonval type metermovement to which the invention relates;

FIG. 2(a) is a pictorial representation of the permanent magnet andarmature winding of the meter of FIG. 1 illustrating the capacitorplates mounted on the permanent magnet;

FIG. 2(b) is a schematic representation of the capacitors and armaturecoil illustrated in FIG. 2(a) and also showing the electricalconnections; and

FIG. 3 is a schematic representation of the preferred servoed metersystem of the invention.

FIG. 1 illustrates in simplified form a conventional DArsonval typemeter movement to which the invention relates. The magnetic structurecomprises soft iron yoke 1 and permanent magnet 2 suitably arranged toform air gap 3. Coil assembly 4 is movably located within the gap 3, andwhen excited with a control or command signal the signal current and themagnetic field produced by permanent magnet 2 produces a signal torquewhich rotates the armature coil around magnet 2 and displaces thepointer 5 which is mechanically attached to coil assembly 4.

FIG. 2(a) illustrates in more detailed form the permanent magnet andcoil assembly of FIG. 1. Mounted on but electrically insulated from core2 are two shaped metal plates 6 and 7 which preferably are triangular inshape. In actual practice the plates may be formed by depositing copperor other suitable metal on an insulating backing such as epoxy, theepoxy backing being secured to the magnet by any suitable adhesive.Plates 6 and 7 are spaced from one another by gap 8. Coil as sembly 4comprises armature or drive coil 11 and bobbin 9, the bobbin 9comprising a metal, such as copper, suitable for use as a capacitorplate. The meter movement and its hereinafter described control circuitis preferably so constructed that the gap 8 is under the center of thebobbin 9 so that there is equal capacitance between the bobbin and eachof the two capacitor plates when the meter pointer is in the center orzero position; i.e., when no command signal is applied to the drive coil11.

of the two plates to increase as the capacitance v between the bobbinand the other plate decreases.

FIG. 2(b) is an electrical schematic of the coil and capacitorarrangement of FIG. 2(a). The coil bobbin 9 bobbin increases. Thus, byapplying a high frequencyvoltage across bobbin 9 and plates 6 and 7,respectively, the capacitance of the two capacitors C6 and C7 can bedifferentially compared to produce a position signal. The positionsignal produced by this arrangement is desirable over magnetic sensingin that it is collectively more accurate, more linear, more easilycontrolled in production, and is more universally compatible withvarying configurations of the conventional DArsonval meter movement.

FIG. 3 illustrates the meter mechanism of the invention in a preferredembodiment of a closed loop servo control circuit. An input signal, suchas a command signal from the flight director of an aircraft, isamplified by operational amplifier 12 and, applied to meter coil 11. Thesignal current in coil 11 interacts with the unidirectional flux fieldproduced by the permanent magnet 2 and in known fashion causes the metercoil and bobbin 9 to rotate, resulting in deflection of meter pointer 5.An oscillator comprising transistors 15, 16 and 17 and capacitor 20 issupplied by l2 volt supply at 18 and provides a 1 MHz high frequencysaw-tooth voltage output at 19. The high frequency saw-tooth voltage isisolated via transistors 13 and 14 and applied by leads 23 and 24 toplates 6 and 7. Bobbin 9, forming the common plate of the differentiallyvariable capacitors C6 and C7, is connected by lead 37 to a ground orzero volt reference. A feedback network is provided to sense the changein capacitance of capacitors C6 and C7 and comprises leads 23 and 24, adetector circuit comprised of resistors 21 and 22, transistors 13 and 14and diodes 27 and 28, leads 29 and 30, resistors 33 and 34, andcapacitors 35 and 36. The detector circuit and the operational amplifier12 have a 12 volt positive supply voltage applied thereto by lead 40. Abias circuit for regulating the output frequency of the oscillator andthereby maintaining one side of the feedback circuit at zero or groundpotential in a manner hereinafter described comprises transistors 31 and32 and resistor 41. The bias circuit removes most of the common modevoltage from the feedback path. Capacitors and 26 are utilized to filterany residual component of the 1 MHz voltage which may be present in thefeedback path.

In operation, with the system on and no command signal at the input ofthe operational amplifier 12, the needle pointer 5 is at the zero orcenter position of the meter indicator and the coil bobbin 9 is centeredwith respect to gap 8 of plates 6 and 7. The capacitances of capacitorsC6 and C7 are essentially balanced and the feedback to the input ofoperational amplifier 12 is such to drive the pointer 5 to the zeroposition. The emitter current of each of the transistors 13 and 14flowing through resistors 21 and 22 and capacitors C6 and C7,respectively, to ground is proportional to the capacitance of capacitorsC6 and C7 in accordance with the relationship I =(C)(dv/dt), where I isthe emitter current, C is the capacitance, and dv/dl is the positiverate of change of the oscillators output sawtooth voltage with respectto time. Therefore, when a command signal is supplied to the input ofamplifier 12, the meter pointer 5 is deflected in a direction and by amagnitude in accordance with the sense and magnitude of the input signalto a commanded position. Displacement of the needle pointer 5 isaccompanied by a corresponding displacement of bobbin 9 with respect toplates 6 and 7, thereby varying the capacitances between bobbin 9 andplates 6 and 7 resulting in increased emitter current in one oftransistors 13 and 14 and decreased emitter current in the other. Thechange in voltage at the collectors of transistors 13 and 14 changes thefeedback current through resistors 33 and 34 and capacitors 35 and 36,which current is summed at the input of operational amplifier 12 in adegenerative manner to drive the meter to the correct position. Itshould be noted that the capacitors 35 and 36 provide a position ratesignal to give a desired level of damping.

It is desirable to hold the emitter voltage of one of the detectortransistors 13 and 14 to ground potential so the bias circuitaforementioned is provided. In this particular embodiment, the emittervoltage of transistor 14 is held at a zero volt quiescent level byadjusting the oscillator frequency output until the collector current oftransistor 14 is sufficient to give a 12 volt drop across resistor 22,the collector current being proportional to the output frequency of theoscillator. As an example of operation, if the collector voltage oftransistor 14 tends to rise, the transistor 31, having its baseconnected to the collector potential of transistor 14, is turned off,transistor 32 is turned on causing a higher charging rate in capacitor20 and a corresponding increase in the oscillator frequency output. Withthe rise in output frequency of the oscillator, the collector current intransistor 14 is increased, and the tendency of the collector voltage oftransistor 14 to rise is suppressed.

While the plates 6 and 7 have been described in the preferred embodimentas triangular in shape, it is apparent that for different applicationsrectangular plates or other shaped plates may be utilized to obtainother desired feedback characteristic output. Further, while the plateshave been shown mounted on the permanent magnet, they could as well bemounted on the yoke, or in other suitable positions with respect to thebobbin.

While the invention has been described in its preferred embodiment, itis to be understood that changes may be made without departing from thetrue scope of the invention as defined in the appended claims.

We claim:

1. A transducer comprising:

a DArsonval-type meter structure having a magnetic core means arrangedto define a unidirectional magnetic flux field across an air gap;

a conductive bobbin having an armature winding thereon disposed formovement through said field in said air gap; and i first and secondstationary electrically conductive plates mounted on said magnetic coremeans and each having a portion thereof disposed between said conductivebobbin and said magnetic core means such that when said bobbin moves ina given direction the capacitive coupling between said bobbin and one ofsaid plates increases and the capacitive-coupling between said bobbinand the other of said plates decreases.

2. A transducer as in claim 1 wherein said magnetic core structurecomprises a permanent magnet and a surrounding soft iron yoke, and saidplates are mounted on said permanent magnet.

3. A transducer as in claim 1 including means for supplying a directcurrent displacement command signal to said armature winding wherebysaid bobbin and armature winding are displaced through interaction withsaid unidirectional magnetic flux field in accordance with the polarityand amplitudeof said command signal.

4. Apparatus for positioning a meter pointer comprising:

meter movement means of the DArsonval-type meter structure having amagnetic core means arranged to define a unidirectional magnetic fluxfield across an air gap; an electrically conductive bobbin having anarmature winding thereon disposed for movement through said field insaid air gap; two electrically conductive plates mounted on saidmagnetic core means and positioned opposite said bobbin, said bobbin andeach of said plates forming a capacitor, the capacitance of the twocapacitors differentially varying with the movement of said bobbin;means supplying a reversible polarity, variable magnitude direct currentcommand signal to said armature winding for displacing said bobbin and Iservoloop feedback signal proportional to said armature displacement;and

means for summing said command signal and said feedback signal so thatany deviation of the meter from the commanded position is electricallysensed and the error signal is applied to the armature winding to drivethe meter pointer to the correct position.

5. The apparatus of claim 4 wherein said magnetic core structurecomprises a permanent magnet and a soft iron yoke, and said plates aremounted on said permanent magnet core.

6. The apparatus of claim 4 wherein said means for applying a reversiblepolarity, variable magnitude DC command signal to said armature windingincludes an operational amplifier having its output terminal connectedto said armature winding, and wherein said feedback signal and saidcommand signal are summed at the input to said amplifier.

7. The apparatus of claim 6 wherein said position sensing means includesoscillator means for applying a high frequency voltage to each of saidcapacitors;

detector means for developing as its output said servoloop feedbacksignal proportional to said armature displacement; and

bias circuit means for sensing any deviation of one side of saiddetector output voltage from a zero volt quiescent level, the output ofsaid bias circuit being applied to said oscillator to adjust thefrequency of the oscillator output in a direction to drive the detectoroutput voltage back to said zero volt quiescent level.

1. A transducer comprising: a D''Arsonval-type meter structure having amagnetic core means arranged to define a unidirectional magnetic fluxfield across an air gap; a conductive bobbin having an armature windingthereon disposed for movement through said field in said air gap; andfirst and second stationary electrically conductive plates mounted onsaid magnetic core means and each having a portion thereof disposedbetween said conductive bobbin and said magnetic core means such thatwhen said bobbin moves in a given direction the capacitive couplingbetween said bobbin and one of said plates increases and the capacitivecoupling between said bobbin and the other of said plates decreases. 2.A transducer as in claim 1 wherein said magnetic core structurecomprises a permanent magnet and a surrounding soft iron yoke, and saidplates are mounted on said permanent magnet.
 3. A transducer as in claim1 including means for supplying a direct current displacement commandsignal to said armature winding whereby said bobbin and armature windingare displaced through interaction with said unidirectional magnetic fluxfield in accordance with the polarity and amplitude of said commandsignal.
 4. Apparatus for positioning a meter pointer comprising: metermovement means of the D''Arsonval-type meter structure having a magneticcore means arranged to define a unidirectional magnetic flux fieldacross an air gap; an electrically conductive bobbin having an armaturewinding thereon disposed for movement through said field in said airgap; two electrically conductive plates mounted on said magnetic coremeans and positioned opposite said bobbin, said bobbin and each of saidplates forming a capacitor, the capacitance of the two capacitorsdifferentially varying with the movement of said bobbin; means supplyinga reversible polarity, variable magnitude direct current command signalto said armature winding for displacing said bobbin and winding throughthe reaction of said unidirectional magnetic field in accordance withthe sense and magnitude of said commaNd signal; position sensing meansfor differentially sensing the capacitances of said two capacitors todevelop a servoloop feedback signal proportional to said armaturedisplacement; and means for summing said command signal and saidfeedback signal so that any deviation of the meter from the commandedposition is electrically sensed and the error signal is applied to thearmature winding to drive the meter pointer to the correct position. 5.The apparatus of claim 4 wherein said magnetic core structure comprisesa permanent magnet and a soft iron yoke, and said plates are mounted onsaid permanent magnet core.
 6. The apparatus of claim 4 wherein saidmeans for applying a reversible polarity, variable magnitude DC commandsignal to said armature winding includes an operational amplifier havingits output terminal connected to said armature winding, and wherein saidfeedback signal and said command signal are summed at the input to saidamplifier.
 7. The apparatus of claim 6 wherein said position sensingmeans includes oscillator means for applying a high frequency voltage toeach of said capacitors; detector means for developing as its outputsaid servoloop feedback signal proportional to said armaturedisplacement; and bias circuit means for sensing any deviation of oneside of said detector output voltage from a zero volt quiescent level,the output of said bias circuit being applied to said oscillator toadjust the frequency of the oscillator output in a direction to drivethe detector output voltage back to said zero volt quiescent level.